Purdue University
Purdue e-Pubs
International Refrigeration and Air Conditioning
Conference
School of Mechanical Engineering
2008
Fault Detection and Diagnostics for Commercial
Coolers and Freezers
Adam Wichman
Purdue University
James E. Braun
Purdue University
Follow this and additional works at: http://docs.lib.purdue.edu/iracc
Wichman, Adam and Braun, James E., "Fault Detection and Diagnostics for Commercial Coolers and Freezers" (2008). International
Refrigeration and Air Conditioning Conference. Paper 919.
http://docs.lib.purdue.edu/iracc/919
This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for
additional information.
Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/
Herrick/Events/orderlit.html
2313 , Page 1
Fault Detection and Diagnostics for Commercial Coolers and Freezers
Adam Wichman and James E. Braun*
Ray W. Herrick Laboratories
School of Mechanical Engineering
Purdue University
West Lafayette, IN 47907, USA
*Corresponding Author: [email protected]
ABSTRACT
Most of the previous diagnostics research for vapor compression equipment has been performed for packaged air
conditioners and chillers. This paper describes application of a decoupling-based diagnostic technique that was
originally developed for air conditioners to equipment used in walk-in coolers and freezers. Testing was performed
on a small, restaurant-style walk-in cooler and a small walk-in freezer for faulted and un-faulted conditions. The
data was then used to characterize how the fault features respond to faults and to evaluate the performance of the
decoupling-based diagnostic technique.
1. INTRODUCTION
In general, HVAC&R systems are not well maintained [Proctor and Downey (1995), Cowan (2004), Li and Braun
(2007a)] because of the relatively high cost of service and low cost of energy. Recently, a diagnostic method was
developed for vapor compression equipment that handles multiple-simultaneous faults [Li and Braun (2007b)]
through the use of decoupling features [Li and Braun (2007c)]. Decoupling features are parameters that are
uniquely influenced by individual faults and are insensitive to variations in ambient conditions. For example, air
mass flow rate through the condenser is a feature that is strongly influenced by the level of fouling and condenser
fan problems but is nearly independent of other faults that can occur for air conditioning systems that incorporate
fixed-speed fans.
In developing decoupling features, it is important to utilize low cost sensors, such as temperatures. These low-cost
measurements are used in simple models as virtual sensors to infer other system measurements. For example, as
described by Li and Braun (2007c) condenser air flow can be estimated using an energy balance with air and
refrigerant-side measurements. For this energy balance, the refrigerant flow is estimated using a compressor map as
a virtual sensor. Furthermore, virtual evaporating and condensing pressure sensors utilize surface mounted
temperature measurements at locations where saturated conditions exist and property relations to estimate saturation
pressures. The virtual and physical measurements are used to determine the decoupling features for diagnostic
purposes. When a decoupling feature deviates significantly from its normal value then a fault is indicated (e.g., low
condenser air flow for fouling or fan problems).
The decoupling-based FDD method was originally developed for air conditioning (AC) systems. Although the vapor
compression equipment used for commercial refrigerators and freezers are very similar to those used for air
conditioning, operation occurs over a different range of temperatures and the systems utilize different refrigerants.
In addition, commercial refrigeration equipment typically utilize liquid-line receivers which are not generally
employed for air conditioners. In the current paper, the decoupling-based FDD method is applied to equipment used
in small-scale walk-in coolers and freezers. Faults were artificially introduced in the laboratory and the performance
of the diagnostic method was evaluated. A more detailed presentation of the evaluation is presented by Wichman
and Braun (2007).
2. WALK-IN COOLER AND FREEZER EXPERIMENTS
Walk-in cooler and freezer units were tested within psychrometric chambers to allow control of the condenser air
inlet conditions. Figure 1 shows a picture of the cooler unit and walk-in cabinet. The walk-in cooler and freezer
experiments utilized the same refrigerated space but with different refrigeration equipment. The cooler system
utilized R-22 as the refrigerant, whereas the freezer unit employed R-404A. Both systems were equipped with a
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 2
TXV expansion valve and a liquid line receiver. The receiver provides a volume for liquid refrigerant to collect
after it exits the condenser. This volume keeps the refrigerant exiting the condenser in a saturated liquid state during
operation unless the unit is overcharged to the point where the receiver is completely filled with liquid refrigerant.
When the receiver is full, then additional charge added to the system will back up in the condenser and lead to
subcooling of the refrigerant exiting the condenser under steady operating conditions. The liquid line receiver is
shown in Figure1.
Figure 1: Walk-in cooler inside the psychrometric chamber.
Figure 2 shows a schematic of the refrigerant cycle depicting the methods used for simulating faults and the
measurements taken. Refrigerant undercharge, refrigerant overcharge, liquid line restriction, compressor valve
leakage, condenser fouling and evaporator fouling were considered. Compressor valve leakage is meant to include
any fault within the compressor that leads to a loss in volumetric efficiency and refrigerant flow rate. A leaky
compressor valve allows high pressure refrigerant to flow back to the low pressure side of the system which lowers
the volumetric efficiency of the compressor and lowers the mass flow rate of the system. To simulate a loss in
compressor volumetric efficiency, a bypass line with a flow control valve was added around the compressor which
can be opened to allow refrigerant to flow from the high pressure side of the system to the low pressure side. Heat
exchanger fouling in vapor compression equipment can be characterized as a decrease in airflow across the coils.
Previous work [Pak et al. (2005) and Yang et al. (2007)] has demonstrated that the primary effect of air-side heat
exchanger fouling is an increased pressure drop leading to a reduced air flow rate. Fouling was simulated for both
the condenser and evaporator by connecting fan speed controllers to control the air flow rates across the heat
exchangers. During operation, a vapor compression system can experience clogging of the filter dryer which
restricts the flow through the liquid line. In a system with a fixed orifice expansion device, a liquid line restriction
can lead to a reduced mass flow rate. The walk-in cooler tested uses a TXV which can compensate for the pressure
drop incurred for moderate restrictions and keep the refrigerant flow rate relatively constant. For severe restrictions,
the TXV will saturate, opening fully and act like a fixed orifice. A flow control valve was added to the liquid line
which was partially closed to simulate this fault.
For the cooler, each individual fault was simulated at three different ambient conditions (Tcai). Five fault levels were
tested and characterized by percent of nominal cooling capacity. The nominal cooling capacity was determined in
the un-faulted condition or 0th fault level test. Fault levels were chosen so that the degradation in cooling capacity
would occur in even increments with the maximum degradation depending on the fault type. The compressor valve
leakage and liquid line restriction faults degraded cooling capacity by over 50%, and undercharging the system
degraded capacity around 20%. Slowing the heat exchanger fans over the test range degraded the capacity by only
10%. Overcharging the refrigerant had very little effect because of the presence of a receiver. Four combinations of
multiple faults were tested for the cooler. For these tests, the system was undercharged by 50% and four other faults
were applied to the system, all at maximum fault levels defined by the single fault tests.
Based on knowledge gained during testing of the walk-in cooler, a smaller test matrix was employed for the freezer.
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 3
Only one ambient condition was tested. The overcharging tests on the cooler showed that a liquid line receiver
prevents overcharging from being a fault that affects performance of the system. Therefore, no overcharging tests
were performed on the freezer. Multiple fault tests were run on the freezer combining evaporator fouling with the
other four faults. One of the concerns with a freezer is ice accumulation and distinguishing it from evaporator
fouling. To gather data on this phenomenon, an ice accumulation test was performed.
Figure 2: Cooler cycle diagram with added instrumentation.
3. DECOUPLING FEATURES
The decoupling features are based on the work presented by Li and Braun (2007c) and rely on the use of virtual
sensors to achieve low cost with a requirement for only temperature sensors. Table 1 presents the decoupling
features used to diagnose each fault along with the necessary temperature measurements and virtual sensors to
evaluate the features for each fault. Table 2 describes the measurements and models used for the virtual sensors.
As compressor valve leakage (or a similar fault) develops the volumetric efficiency of the compressor decreases.
This decreases the refrigerant mass flow rate, which in turn, decreases the discharge enthalpy. In order to detect this
type of fault, a simple model is used to estimate the nominal discharge temperature (Tdis) from the compressor. This
expected value is then compared to the measured Tdis. The normal discharge enthalpy is calculated using a steadystate energy balance on the compressor including power input, heat loss, and enthalpy change of the refrigerant flow
stream. The refrigerant flow, power consumption and heat loss are estimated using virtual sensors as outlined in
Table 2.
A liquid line restriction fault is meant to represent a dirty filter dryer. In a typical air conditioning system, there is
usually about 15°F of condenser subcooling. This makes it difficult to use a temperature drop across the filter dryer
(ǻTll) as a fault indicator because it would require a large pressure drop to get the refrigerant to change phase and
generate a large enough ǻTll for detection. However, for systems with a liquid line receiver, the refrigerant at the
condenser exit is a saturated liquid. Therefore, it requires a very small pressure drop in the liquid line to get the
refrigerant to change phase. For detection of a liquid-line restriction, using two temperature measurements across
the filter dryer is adequate since a change in temperature can be directly correlated to pressure drop.
Condenser fouling occurs as a result of dirt and debris, whereas evaporator fouling is meant to represent a dirty filter
or a frost accumulation. The primary impact of any of these fouling types is a reduction in air flow. In order to
detect fouling faults, virtual sensors for air volumetric flow rate are employed. The air flow rates are determined
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 4
using energy balances on the condenser and evaporator using temperature and virtual measurements. The effect of
moisture removal is neglected for the evaporator since the humidity ratios and moisture removal rates are low at the
low temperatures associated with cooler and freezer applications.
The difference between the superheat at the evaporator exit and the subcooling at the condenser exit is used as the
decoupling feature to detect high or low refrigerant charge. This feature is incentive to charge until the liquid-line
receiver is either empty or full. When the system is undercharged to the point where the receiver is empty, there
should be two-phase refrigerant in the liquid line. This causes the TXV to fully open and lose control of the
superheat. Undercharging the system further should cause superheat to increase while subcooling should remain
approximately equal to zero. At the other extreme, when the receiver is completely full then refrigerant will back up
in the condenser and additional charge will cause an increase in the condenser subcooling. However, the superheat
should remain relatively constant for overcharging. Since superheat only changes for low refrigerant charge and
subcooling only varies for severe overcharging, the simple decoupling feature is reasonable
Table 1: Fault decoupling features and measurement requirements
Fault
Compressor Valve Leakage
Decoupling
Feature
Tdis-Tdis,normal
Temperature
Measurements
Tdis, Tsuc
Liquid Line Restriction
Condenser Fouling
TLL,1-TLL,2
V
TLL,1, TLL,2
Tcai, Tcao, Tdis, Tcond,out
Pcond,in, Pcond,out, m ref
Evaporator Fouling or
Frost Accumulation
Low or High
Refrigerant Charge
Vea
Teai, Teao, Tsuc, Tcond,out
Psuc, Pcond,out, m ref
Tsh-Tsc
Tsuc, Tcond,out
Psuc, Pcond,out
ca
Virtual Measurements
Psuc, Pcond,in, m ref , Wcomp , Q comp ,loss
-
Table 2: Virtual sensors measurement and modeling requirements
Virtual
Sensor
Psuc
Temperature
Measurements
Tevap,in
Virtual
Measurements
-
Pcond,in,
Pcond,out
Tcond
-
m ref
Tsuc
Psuc , Pcond,in,,
Wcomp , Q comp ,loss
Wcomp
Tevap,out
Psuc , Pcond,in,
Q comp ,loss
Tsuc, Tdis,Tamb
-
Model
Saturation pressure for refrigerant evaluated at the
refrigerant temperature entering the evaporator (Tevap,in).
Assumes negligible pressure drop across the evaporator.
Insulated surface temperature measurement is sufficient.
Saturation pressure for refrigerant evaluated at the
temperature of a return bend within the two-phase section
of the condenser (Tcond). Assumes negligible pressure
drop across the condenser. An insulated surface
temperature measurement is sufficient. For the cooler and
freezer in this study, the fifth return was found to work
well for the condenser.
Compressor performance map for refrigerant flow rate in
terms of inlet pressure (Psuc) and temperature (Tsuc) and
outlet pressure (Pcond,in). Alternatively, an overall energy
balance is employed when a compressor leakage fault has
been identified.
Compressor performance map for power in terms of inlet
pressure (Psuc) and temperature (Tsuc) and outlet pressure
(Pcond,in). The power is relatively insensitive to a
compressor leakage fault.
An empirical correlation for compressor heat loss rate.
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 5
4. FAULT FEATURE CHARACTERISTICS
In this section, individual fault features are presented as a function of percent capacity degradation associated with
different individual faults. The cooling capacity of the 0th fault level was taken as a baseline value for determining
percent capacity degradation. Cooling capacity degradation was chosen as a general indicator of the severity of fault
because it is more sensitive to fault level than efficiency degradation. Furthermore, for refrigeration equipment, lost
cooling capacity could mean the loss of valuable refrigerated or frozen product, which is more important than lost
efficiency. Only sample results are presented in this section that highlight particular performance characteristics and
problems. The fault features were calculated for two cases: 1) using all of the measured data and 2) using
temperature sensors only with virtual sensors.
4.1 Compressor Valve Leakage
Discharge Temperature Residual [F]
Figures 3 and 4 show the effects of capacity degradation due to compressor valve leakage on discharge temperature
residuals for the walk-in cooler and freezer. This fault feature increases with fault level for both systems with either
real or virtual sensors. However, the discharge residual is much less sensitive to capacity degradation for the cooler
than for the freezer. The small dependence of discharge temperature residual on capacity degradation for
compressor valve leakage fault with the cooler is problematic because this feature also depends on condenser
fouling. This effect may be due to an inadequate heat loss model for the compressor. The compressor heat loss is
much more significant for the cooler application than for the freezer application because of higher suction pressures
and temperatures. Therefore, model errors have a more significant effect on discharge temperature for the cooler.
30
Real Sensors 55F
Real Sensors 75F
Real Sensors 95F
20
Virtual Sensors 55F
Virtual Sensors 75F
Virtual Sensors 95F
10
0
-10
-10
0
10
20
30
40
50
60
Percent Capacity Degradation due to Compressor Valve Leak
[%]
Discharge Temperature Residual [F]
Figure 3: Tdis,residual vs. % capacity degradation due to compressor valve leakage for the walk-in cooler.
50
40
30
20
10
0
Real Sensors 75F
Virtual Sensors 75F
-10
-20
-1
1
3
5
7
9
Percent Capacity Degradation due to Compressor Valve Leak
[%]
Figure 4: Tdis,residual vs. % capacity degradation due to compressor valve leakage for the walk-in freezer.
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 6
The compressor valve leakage fault also caused the feature for low refrigerant charge to increase at high fault levels
for the walk-in cooler. As the compressor flow is reduced, the TXV must open to maintain the target superheat. At
severe fault levels, the TXV saturates open and the superheat increases. This could potentially lead to a false alarm
for low charge for capacity degradations greater than about 40%. However, this may not be a problem because the
compressor fault would normally be identified at lower fault levels before the charge feature was affected.
Furthermore, false alarms could be avoided for individual faults because the diagnostic feature for compressor valve
leakage would take precedence. In other words, if the compressor temperature discharge feature exceeded a
threshold for this fault then the refrigerant charge feature would be ignored. However, under some circumstances it
may not be possible to remotely diagnose both low refrigerant charge and compressor valve leakage if they were
simultaneously present. In this case, a technician might need to visit the unit and use a sight glass to determine
whether the refrigerant charge were low prior to deciding whether to replace the filter drier.
4.2 Condenser and Evaporator Fouling
Figures 5 and 6 show the impacts of condenser and evaporator fouling level on virtual condenser and evaporator air
flow rates. These features are very sensitive to the level of fouling and decrease by about a factor of two for about a
5% degradation in cooling capacity. Very similar results were obtained for the freezer. Furthermore, these features
were insensitive to the other faults considered.
Virtual Condenser Air Flow Rate
[ft^3/min]
900
Real Sensors 55F
Real Sensors 75F
Real Sensors 95F
800
700
Virtual Sensors 55F
Virtual Sensors 75F
Virtual Sensors 95F
600
500
400
300
200
100
0
-1
0
1
2
3
4
5
6
7
8
9
Percent Capacity Degradation due to Condenser Fouling [%]
Virtual Evaporator Air Flow Rate [ft^3/min]
Figure 5: Virtual condenser air flow rate vs. % capacity degradation due to condenser fouling for walk-in cooler.
1000
Real Sensors 55F
Real Sensors 75F
Real Sensors 95F
900
800
700
Virtual Sensors 55F
Virtual Sensors 75F
Virtual Sensors 95F
600
500
400
300
200
100
0
-1
1
3
5
7
9
11
Percent Capacity Degradation due to Evaporator Fouling [%]
Figure 6: Virtual evaporator air flow rate vs. % capacity degradation due to condenser fouling for walk-in cooler.
One of the issues for the walk-in freezer is to be able to distinguish evaporator fouling from ice accumulation on the
heat exchanger prior to a defrost cycle. To characterize the ice buildup, an ice accumulation test was run by
introducing a moisture source inside of the freezer compartment. Figure 7 shows the output from the virtual
evaporator air flow rate sensor over time during the ice build up. This is a good indicator of the frost accumulation
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 7
and could be used as part of a “smart” defrosting scheme to determine the need for defrosting. Although not shown
in this figure, the virtual evaporator air flow feature returned to a normal value of about 1200 cfm after a defrost
cycle. As a result, evaporator fouling and ice accumulation could easily be distinguished within a fault detection
system. After a defrost cycle and when steady state conditions are achieved, the evaporator air flow rate should
return to a normal, un-faulted value. If the evaporator were fouled, then a defrost cycle would not return the virtual
evaporator air flow indicator to normal and a fault could be flagged.
Virtual Evaporator Air Flow Rate [ft^3/min]
1400
1200
1000
800
600
400
200
0
0
2000
4000
6000
8000
10000
Time [s]
Figure 7: Virtual evaporator over the course of the ice accumulation test.
4.3 Liquid Line Restriction
Liquid Line Temperature Difference [F]
Figure 8 shows the effect of capacity degradation due to liquid line restriction on the liquid line temperature
difference for the cooler. This fault feature increases significantly with fault level for all three ambient conditions.
However, the impact is less at lower ambient temperatures due to a lower pressure ratio for the system. For the
same fault level, the pressure drop across the restriction is larger for higher ambient temperatures, which increases
the temperature drop across the restriction. Very similar results were obtained for the freezer.
60
Real Sensors 55F
Real Sensors 75F
Real Sensors 95F
50
40
30
20
10
0
-10
-10
0
10
20
30
40
50
60
Percent Capacity Degradation due to Liquid Line Restriction [%]
Figure 8: Liquid line temperature difference vs. % capacity degradation due to a liquid line restriction for the walkin cooler.
The liquid line restriction also caused the feature for low refrigerant charge to increase. This occurs because of an
interaction with the TXV. As the refrigerant flow is reduced with a liquid line restriction, the TXV must open to
maintain the target superheat. At a sufficient fault level, the TXV saturates open and the superheat increases.
Although the refrigerant charge feature is sensitive to liquid line restriction, false alarms can be avoided for
individual faults because the diagnostic feature for liquid line restriction would take precedence. In other words, if
the liquid line feature exceeded a threshold for this fault then the refrigerant charge feature would be ignored.
However, it may not be possible to remotely diagnose both low refrigerant charge and a liquid line restriction if they
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 8
were simultaneously present. In this case, a technician would need to visit the unit and use a sight glass to determine
whether the refrigerant charge were low. A liquid line restriction did not cause changes in other fault features that
would lead to false alarms.
4.4 Refrigerant Charge
Figure 9 shows the effect of refrigerant charge level on capacity degradation for the walk-in cooler. Overcharging
the system has very little effect on performance because of the liquid line receiver. There is a small impact of very
high charge levels at low ambient temperatures at the point where the receiver becomes full of refrigerant. Reduced
charge does have an impact on cooling capacity once the charge is less than about 80% of the nominal charge. At
this point, the liquid line receiver is empty and the TXV saturates at a fully open position. This leads to an
increasing superheat and fault feature with decreasing charge as demonstrated in Figure 10. Similar results were
obtained for the freezer. Low or high refrigerant charge did not have a significant effect on other fault features and
would not lead to false alarms. Although it is not possible to detect system overcharging for these systems, this is
not a problem because overcharging does not impact system performance.
Percent Degradation of Capacity [%]
25
Real Sensors 55F
Real Sensors 75F
Real Sensors 95F
20
15
10
5
0
-5
0
50
100
150
200
250
Charge [%]
Difference of Suction Line Superheat &
Liquid Line Subcooling [F]
Figure 9: Percent capacity degradation as a function of charge for the walk-in cooler.
35
30
25
20
15
10
Real Sensors 55F
Real Sensors 75F
Real Sensors 95F
5
Virtual Sensors 55F
Virtual Sensors 75F
Virtual Sensors 95F
0
0
50
100
150
200
250
Charge [%]
Figure 10: Difference in Tsh and Tsc as a function of percent nominal charge for the walk-in cooler.
5. OVERALL DIAGNOSTIC PERFORMANCE
Fault features determined using data from the experiments were used along with fault thresholds to determine the
levels at which faults could be detected and to identify false alarms. Thresholds for each of the decoupling features
were chosen to correspond to a 5% degradation in cooling capacity. Virtual sensors were used for this evaluation to
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 9
represent actual implementation of the decoupled diagnostic method. Tables 3 and 4 give results for fault detection
sensitivity. For the single fault tests on both the walk-in cooler and freezer there were no missed faults. The
diagnostic method was able to detect compressor valve leak, condenser fouling, and liquid line restriction at the first
fault level implemented on the cooler. Liquid line restriction was detected at the first level on the freezer as well.
Table 3: Fault levels where the algorithm could first diagnose individual faults for the walk-in cooler.
Fault
Compressor Valve Leakage
Compressor Valve Leakage
Compressor Valve Leakage
Condenser Fouling
Condenser Fouling
Condenser Fouling
Evaporator Fouling
Evaporator Fouling
Evaporator Fouling
Liquid Line Restriction
Liquid Line Restriction
Liquid Line Restriction
System Undercharge
System Undercharge
System Undercharge
Ambient
Temperature [F]
55
75
95
55
75
95
55
75
95
55
75
95
55
75
95
Fault Level
1st
1st
1st
1st
1st
1st
2nd
3rd
3rd
1st
1st
1st
4th
3rd
3rd
Degraded Capacity
[%]
21.0
16.0
17.5
1.2
1.2
2.6
1.9
5.2
6.1
19.4
7.5
8.5
7.9
10.5
6.1
Table 4: Fault levels where the algorithm could first diagnose individual faults for the walk-in freezer.
Fault
Ambient
Temperature [F]
Fault Level
Degraded
Capacity [%]
Compressor Valve Leak
Evaporator Fouling
Liquid Line Restriction
System Undercharge
75
75
75
75
3rd
3rd
1st
4th
3.5
3.0
22.5
11.9
6. CONCLUSIONS
In general, the decoupling features developed for compressor valve leakage, condenser fouling, evaporator fouling,
liquid line restriction faults, and refrigerant charge should work well for diagnostics applied to vapor compression
systems used in walk-in coolers and freezers. However, it was found that the refrigerant charge feature is not
completely decoupled from liquid line restriction and compressor valve leakage faults at high fault levels. Systems
that utilize a liquid line receiver and TXV are insensitive to charge until the system becomes starved and the TXV
saturates open. A saturated TXV leads to an increase in superheat which is used to indicate low refrigerant charge.
However, the TXV can also be saturated open when refrigerant flow is reduced significantly due to a high level of
compressor valve leakage or a severe liquid line restriction.
Although the refrigerant charge feature is sensitive to liquid line restriction and compressor valve leakage faults,
false alarms can be avoided for individual faults because the diagnostic features for these other faults work well and
would identify the appropriate fault in the absence of low charge. However, it is not possible to identify multiple
fault combinations of low refrigerant charge and either compressor valve leakage or liquid line restriction.
Furthermore, it is not possible to determine overcharging of refrigerant with this feature unless the charge is severely
overcharged to the point where the receiver is full. For systems with liquid line receivers and TXVs, it may make
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008
2313 , Page 10
sense to employ a low-cost level sensor in the receiver. The normal charge level could be correlated with operating
conditions and used to decouple charge from other faults.
There is also a need to develop a better model for estimating heat loss in order to improve estimates of the normal
compressor discharge temperature under all conditions. This should eliminate an indication of compressor value
leakage when condenser fouling is present.
Evaporating frosting tests were also implemented and the evaporator fouling feature was found to be a reliable
indicator. This feature could be used as part of a “smart” defrosting scheme to determine the need for defrosting. A
threshold could be determined that balances tradeoffs between improved cycle efficiency and defrost energy. This
could lead to reduced defrost cycles for situations where the evaporator is not exposed to significant moisture (e.g.,
infrequent door openings) and increased defrost cycles for cases with high indoor box moisture levels.
NOMENCLATURE
m ref
refrigerant mass flow rate
Pcond,in
condenser refrigerant inlet pressure
Pcond,out,
Q comp ,loss
condenser refrigerant outlet pressure
compressor heat loss rate
Psuc
Tamb
compressor inlet pressure
compressor ambient temperature
Tcai
Tcond
Teai
Tevap,in,
Tdis,normal
TLL,2
Tsc
Vca
W
condenser air inlet temperature
representative refrig. condensing temp.
evaporator air inlet temperature
evaporator refrigerant inlet temperature
normal compressor discharge temperature
temperature at outlet of filter drier
subcooling at condenser outlet
virtual condenser air flow rate
Tcao
Tcond,out
Teao
Tdis
TLL,1
Tsh,
Tsuc
Vea
condenser air outlet temperature
condenser refrigerant outlet temperature
evaporator air outlet temperature
compressor discharge temperature
temperature at inlet of filter drier
superheat at evaporator outlet
compressor inlet temperature
virtual evaporator air flow rate
comp
compressor power
REFERENCES
Cowan, A., Review of recent commercial rooftop unit field studies in the Pacific Northwest and California.
Northwest Power and Conservation Council and Regional Technical Forum, Portland, Oregon, October 8, 2004
Li, H. and Braun, J.E., “Evaluation of a decoupling-based fault detection and diagnostic technique – Part II: Field
evaluation and application”, Journal of Harbin Institute of Technology, 2007a.
Li, H. and Braun, J.E., “A methodology for diagnosing multiple-simultaneous faults in vapor compression air
conditioners”, International Journal of Heating, Ventilating, Air-Conditioning and Refrigerating Research, vol.
13, no. 2, pp. 369-395, 2007b.
Li, H. and Braun, J.E., “Decoupling features and virtual sensors for diagnosis of faults in vapor compression air
conditioners”, International Journal of Refrigeration, Vol. 30, Issue 3, PP. 546-564, 2007c.
Pak, B.C., Groll, E.A., and Braun, J.E., 2005, Impact of fouling and cleaning on plate fin and spine fin heat
exchanger performance, ASHRAE Transactions, Vol. 111, Part 1, pp. 496-504
Proctor, J. and Downey, T., Heat pump and air conditioner performance. Handout from oral presentation. Affordable
Comfort Conference, Pittsburgh, PA, March 26-31, 1995.
Wichman, A.J. and Braun, J.E., “Evaluation of Fault Detection and Diagnosis Methods for Refrigeration Equipment
and Air-Side Economizers”, Report HL 2007-8, Ray W. Herrick Laboratories, Purdue University, 2007.
Pak, B.C., Groll, E.A., and Braun, J.E., 2005, Impact of fouling and cleaning on plate fin and spine fin heat
exchanger performance, ASHRAE Transactions, Vol. 111, Part 1, pp. 496-504
Yang, L., Braun, J.E., and Groll, E.A., 2007, The impact of fouling on the performance of filter-evaporator
combinations, International Journal of Refrigeration, Vol. 30, No. 3, pp. 489-498.
ACKNOWLEDGEMENT
This work was supported by the Department of Energy (DOE). We appreciate the donation of equipment provided
by Manitowoc, Inc. and coordinated by Daryl Erbs.
International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008